When an object such as an airplane travels through air, the movement of the plane causes a pressure disturbance that moves at the speed of sound. By way of example, the sound waves created by the nose of the plane will travel in an outward direction away from the aircraft. The air ahead of the plane receives the sound waves before the arrival of the aircraft, so that when the aircraft arrives, the air flows around the plane. As the airplane approaches the speed of sound, the sound waves merge closer and closer together at the front of the plane. When the aircraft is moving at the speed of sound the sound waves merge together into a “shock wave” which is an almost instantaneous line of change in pressure, temperature and density.
An aircraft traveling at supersonic speed will generate a bow shock wave from the nose of the airplane and a tail shock wave created by the tail of the plane. The tail shock wave is created by an under pressurization in the air about the tail. The overall shock wave pressure gradient extends from an over-pressure area beneath the forward portion of the plane and an under-pressure area beneath the aft section of the aircraft. Pressure disturbances generally coalesce into an N-wave shape that has the largest shock magnitudes at the inflection points of a pressure gradient curve. Since the front of a supersonic aircraft generates an increase in ambient pressure, and the rear generates a decrease in pressure, the variation in propagation speed causes aircraft pressure disturbances to stretch out as they propagate to the ground. As the disturbances stretch out, they also tend to coalesce because shocks waves travel halfway between the speed of the lower pressure ahead and higher pressure behind.
The shock waves travel through the atmosphere to the ground. To an observer, the shock waves are felt as an abrupt pressure compression followed by gradual decompression and ending in an abrupt pressure compression back to ambient. The abrupt compressions (also known as shocks) create a disturbingly loud sound. Additionally, the shock waves may cause structural damage to surrounding buildings.
Sonic booms are often measured in pounds per square foot (psf) of overpressure. Overpressure is the increase over normal atmospheric pressure (2,116 psf). One pound of overpressure would not be expected to produce damage to structures. Sonic booms may cause minor damage such as shattered glass but structurally sound buildings should not suffer damages from overpressures less than 16 psf. Sonic boom exposure to communities typically does not exceed 2 psf. Some public reaction could be expected between 1.5 and 2 psf. Rare minor damage may occur with 2 to 5 pounds overpressure.
Supersonic flight over land by civil aircraft is prohibited in the United States. The current regulations applicable to supersonic aircraft are found in 14 CFR part 36, Subpart D, “Noise Limits for Supersonic Transport Category Airplanes,” and 14 CFR part 91, Subpart I, “Operating Noise Limits.” The regulations require that the noise levels of the airplane must be reduced to the lowest levels that are economically reasonable, technologically practicable, and appropriate for a supersonic design. Part 91 prohibits civil aircraft operation at greater than Mach 1 over the United States. Part 91 also imposes flight limitations to ensure that civil supersonic flight entering or leaving the United States will not cause a sonic boom to reach the surface within the United States. Supersonic Transports (SSTs) are therefore restricted to supersonic flight across water, thereby limiting the usefulness of the planes.
In 1990, the FAA proposed to amend the type certification noise standards and noise operating rules for future generation civil supersonic airplanes. After analyzing the comments received on the Notice of Proposed Rulemaking (NPRM), the FAA determined that further investigation and research was necessary before a final rule could be developed. Accordingly, the FAA withdrew the proposed rule and instead issued a policy on noise issues involving the development of future generation civil supersonic transport airplanes. It is likely that new regulations will be adopted in the future that allow supersonic flight over the United States provided that the sound pressure levels caused by the aircraft at ground level are within an acceptable limit.
Shock waves, and thus sonic booms, are fundamental to supersonic flight and can be minimized, but not eliminated, on aircraft that generate lift forces during flight. A significant finding from past sonic boom studies is that startle, rattle, and building vibrations (which can cause damage) are key elements in determining the response of the public to sonic booms. Pressure disturbances of less than 1.0 lb/ft2 will produce less startle, rattle, and building vibrations. NASA's High Speed Research Program identified three key requirements for overland supersonic flight: (1) establishing the criteria for an acceptable “shaped” sonic boom signature, (2) designing a viable aircraft to produce that shaped signature, and (3) quantifying the influence of the atmosphere on such signatures.
With the likelihood of supersonic flight being allowed over the United States in the future, it is desirable to provide systems to alert crewmembers of the level of sonic boom disturbance that have been caused, and are likely to be caused, under current flight conditions. It is also desirable to provide cues to the crewmembers indicating modifications to the flight condition that could lessen the severity of the disturbance. In some circumstances, it is also desirable to limit a pilot's ability to execute maneuvers that would cause sonic boom disturbances above a predetermined level, except under certain situations, such as emergency conditions. Additionally, engine and airframe noise around the airport and during climb could also be handled by such a system to improve airport and community noise abatement.